Unless there's a feature Catalina has and Mojave does not that you can't live without out, you're much better off running a Mojave/HFS+ setup on that 2009. It'll be much faster, and run 32bit apps.. (If hardware-acceleration will not enable on the incapable machine, then backgrade further to High Sierra, and install Chromium-legacy as a modern browser.)
First you need to start over and create a new installer. Certificates, permissions, etc expire. Make a fresh one.
Your disk needs to be formatted as HFS and NOT APFS. High Sierra was the first OS to be able to use APFS.
We may be in danger of being sidetracked, maybe some misunderstanding, but we are talking about Catalina 10.15 no? Catalina, 2 OS versions after High Sierra 10.13. I'm about to replace the internal HDD with an SSD, but in addition to running High Sierra 10.13.6, have been running Catalina 10.15.7 (via dosdude patcher) formatted APFS on a separate partition for at least the last 3 years.
Sorry, you are absolutely correct - it's so long since I used Catalina that I forgot it came after both High Sierra and Mojave.
Thanks already saw that. But what remains puzzling is that before cloning/restoring my dosdude 10.15.7 to the new SSD, this never used to happen with Firefox - only with Brave or Chrome. In fact, never could get Brave or Chrome to run at all. Maybe a complete reinstall will go the trick, though not sure it's worth all the trouble.
Realize that this is from an old post, but on my 10.15.7, (Late '09 iMac 10,1) ran this suggested command: sudo nvram boot-args=amfi_get_out_of_my_way=0x1 and, while it allowed Firefox to open on the first launch, it rather messed up the settings in my 128.3esr, which now show misaligned checkboxes, as well as very elongated settings items, which had previously been fixed with CSS, but which now has no effect at all.
